Merge pull request #474 from System-IO-Abstractions/dependabot/nuget/nunit-3.12.0

Bump nunit from 3.11.0 to 3.12.0

Author: robkeim

System.IO.Abstractions.TestingHelpers.Tests/System.IO.Abstractions.TestingHelpers.Tests.csproj

@@ -46,7 +46,7 @@
   </ItemGroup>
 
   <ItemGroup>
-    <PackageReference Include="nunit" Version="3.11.0" />
+    <PackageReference Include="nunit" Version="3.12.0" />
     <PackageReference Include="NUnit3TestAdapter" Version="3.13.0" />
   </ItemGroup>